Various types of illumination systems for illuminating objects are well-known in the art and may be used in a wide range of applications. One specific type of illumination system relates to systems for illuminating elongate objects having large length-to-width (i.e., aspect) ratios. Examples of such large aspect ratio objects include certain types of bar-code labels or other elongate labels having machine-readable indicia provided thereon. While devices for illuminating and reading data from such machine-readable (e.g., bar code) labels are known and are being used, certain types of bar code reading systems may encounter problems if the length of the bar code label exceeds the design limit length of the bar code reader. For example, portions of the machine-readable label that are located near or outside the design limit length of the reader may be improperly or insufficiently illuminated by the illumination system associated with the reader. This can make it difficult for the reader to reliably detect the machine-readable information located in the improperly- or insufficiently-illuminated areas.
The illumination problem may be better illustrated by example. Consider the situation illustrated in FIG. 1 involving an illuminated object O having a length L, the image of which is focused onto the surface of a detector D by a lens S. In order for the detector D to capture the desired detail of the object O along its entire length L, all portions of the illuminated object along its length L should equally illuminate the detector D, or at least illuminate the detector D within an allowable detection range. Unfortunately, however, the illumination, i.e., the density of luminous flux, on the surface of detector D varies inversely with the square of the distance from the illuminated object O (a luminous source) and directly with the cosine of the angle θ between the luminous flux and the normal N to the surface of detector D. Therefore, the illumination on the surface of the detector D decreases toward each end of the object O, and some means for compensating for this reduction in illumination must be found if the detector D is to have the same effective sensitivity for points near each end of the illuminated object O as it does for points near the center of the object O.
The illumination problems described above tend to be even more severe if the light source used to illuminate the object does not uniformly illuminate the object along its length. For example, if the object is illuminated with light from a single lamp, the illumination provided to the object will also decrease in the manner described above, i.e., with the square of the distance between the light source and the object and directly with the cosine of the angle between the lamp and the normal to the surface of the object. Accordingly, the illumination loss at the detector when detecting objects having relatively large aspect ratios (i.e., ratios of length-to-width) is due to two factors: 1) the increased distance that such “off-center” points are located from the detector; and 2) the fact that such off-center points receive less illumination from the light source.